1. Field of the Invention
This invention relates to inorganic luminescent compositions, and more particularly, to compositions including silicate of alkaline earth materials which are modified by at least one halide.
2. Description of the Related Art
Phosphorescent materials are widely known, and applied for a variety of purposes. Many of these purposes are consumer oriented, others are industrial in nature. Given the diversity of applications, there is an ongoing desire for improved phosphorescent materials.
Phosphors, as discussed herein, include materials that exhibit luminescence. Such materials emit light when excited with external pumping light. The emission originates from a small amount of activators incorporated within the crystal lattice. This small fraction is referred to as “emission centers,” “luminescence centers,” and by other similar terms. Phosphors are usually made from a suitable host material with an added activator, and are formed into the crystal structure. Such activators in the crystal structure are usually introduced in trace amounts, and give rise to the emission.
The crystal structure of a material or the arrangement of atoms within a given type of crystal structure can be described in terms of its unit cell. In simple terms, the unit cell is a small box containing one or more atoms in a spatial arrangement with certain symmetry. When stacked in a three-dimensional space, the unit cells describe the bulk arrangement of atoms of the crystal. The unit cell is given by its lattice parameters which are the length of the cell edges and the angles between them, while the positions of the atoms inside the unit cell are described by a set of atomic positions measured from a lattice point. A variety of lattice systems are known, and include, for example, triclinic, monoclinic, orthorhombic, rhombohedral, tetragonal, hexagonal, and cubic systems. Many of these crystal structures have variations as well.
Each of the atoms bound in the crystal has certain electronic properties (i.e., energy levels or shells in which its respective electrons may reside). The electron shells are labeled K, L, M, N, O, P, and Q; or 1, 2, 3, 4, 5, 6, and 7; going from the innermost shell outwards. Electrons in outer shells have higher average energy and travel farther from the nucleus than those in inner shells. This makes them more important in determining how the atom reacts both chemically and physically and behaves as a conductor, among other things. Each shell is composed of one or more subshells, which are themselves composed of atomic orbitals. For example, the first (K) shell has one subshell, called “1s”; the second (L) shell has two subshells, called “2s” and “2p”; the third shell has “3s”, “3p”, and “3d”; and so on. Each of the electron shells are filled according to certain theoretical constraints. For example, each s subshell holds at most two electrons; each p subshell holds at most six electrons; each d subshell holds at most ten electrons, and so on.
Once the atoms are bound in the crystalline structure, electrons may be shared between the atoms. The crystalline structure provides for additional and unique electronic properties as a result of the particular chemical bonds and defects formed therein. Accordingly, for each unique phosphor, the specific composition and structure of the phosphor provides unique combinations of energy levels from which electrons may decay, and therefore emit photons.
The host materials are often one of an oxide, nitride, oxynitride, sulfide, selenide, halide, and may include calcium, strontium, aluminium, silicon, or various rare earth metals. Among other things, the activators may prolong the emission time (afterglow) of the phosphor once it has been excited by a light source.
Fabrication of a phosphor generally involves a multi-step process. In this process, bulk material (which includes the host material, as well as the activator, and may contain other materials, such as a co-activator and flux) is milled to a desired particle size. The mixture is then fired for some period of time at temperatures ranging from about 900 degrees Celsius to about 1,500 degrees Celsius or more. Most often, the firing involves environmental controls, such as the use of a desired gas, or mixture of gases. The process may be repeated if desired. Although firing of the mixture is used to form well crystallized particles, a flux compound may be added to assist this formation. Generally, the addition of flux reduces the energy required for crystal growth.
Although some materials may be beneficial for achieving desired properties in the formation of a phosphor, there is a risk that incorporation of excessive quantities or improper forms of materials will ultimately quench output of the finished product.
The commonly quoted parameters assessing performance of a phosphor include the wavelength of emission maximum (in nanometers, or alternatively color temperature in degrees kelvin, such as for white blends), the peak width (in nanometers, usually at fifty percent of intensity, or full-width, half-maximum (FWHM)), and decay time (in seconds).
Accordingly, improvement to phosphors requires careful consideration of many aspects of crystal structures as well as thoughtful fabrication techniques.
Given the continuing need for phosphorescent materials that meet demanding standards of use, there are many opportunities to provide improved phosphor materials. Preferably, the phosphorescent materials perform well optically (such as by exhibiting high intensity of emissions as well as by exhibiting persistent emissions). It is also preferred that the phosphorescent materials exhibit emissions in wavelengths (or bands of wavelengths) that have not been previously exhibited, or exhibited by optically robust materials.